1. Field of the Invention
This invention relates to a method for the formation of a hydrophilic corrosion-resistant coating on the surface of a metallic material, particularly a material of aluminum or aluminum alloy (hereinafter referred to collective as "aluminum").
2. Description of the Prior Art
Metallic materials, particularly aluminum material, excel in thermal conductivity and, therefore, have long been used extensively in making fins for heat exchangers in air conditioners. When the metallic materials are used in heat exchangers, the properties they are required to possess are freedom from corrosion under wet conditions and sufficient surface hydrophilicity to be readily wet with water.
Heretofore, in the use of aluminum for fins in heat exchangers, it has been often customary for the aluminum material to be chemically treated by the so-called chromate method, which comprises immersing the aluminum material in an aqueous solution of a chromate, or by the so-called phosphoric acid chromate method, which comprises immersing the aluminum material in the aqueous solution containing a chromate, a phosphate, and a fluoride thereby forming a corrosion-resistant protective coating on the surface of the aluminum material before the aluminum material is exposed to a corrosive environment. Although the coating obtained by the conventional chemical treatment is relatively high in corrosion-resistance, its surface is deficient in hydrophilicity. When this material is used for cooling fins, for example, the fins cause poor cooling efficiency because the water from the ambient air which condenses on the surface of the coating does not uniformly wet that surface but collects as spherical drops. Further, these condensed droplets on the surface of fins often form what is called a bridge between fins and, as the result, offers increased resistance to the current of air flowing through the air conditioner and causes noise and increased energy consumption.
In order to confer hydrophilicity upon the corrosion-resistant coating formed chemically on the surface of a metallic material. U.S. Pat. No. 4,462,842 discloses a method which comprises immersing an aluminum material on which a chromate or chromate-phosphate coating has been formed in a bath containing colloidal silica or silicate, thereby depositing on the coating a hydrophilic colloidal silica or silicate layer which imparts hydrophilicity to the coating. The hydrophilic colloidal silica or silicate layer thus formed on the surface of the chromate or chromate-phosphate coating has insufficient binding force or adherence for the chromate or chromate-phosphate undercoating and, therefore, has the disadvantage that it will readily peel off the surface of the coating while in use and is not durable.
In view of this state of affairs and with a view to forming a more durable hydrophilic coating on the surface of a metallic material, the inventors previously proposed a method for the formation of a coating (Japanese Patent Application Laid-open No. SHO 60(1985)-39,169) which comprises coating the surface of a metallic material with a treating liquid prepared by adding a water-soluble acrylic acid polymer and colloidal silica to an aqueous solution containing trivalent and hexavalent chromium compounds and a fluorine compound, drying the coated metallic material, and subjecting the dried coated metallic material to a baking treatment. In accordance with this method, a durable hydrophilic coating can be formed because the phosphoric chromate type undercoat formed primarily on the surface of the metallic material is covered with an acrylic resin coating vested with hydrophilicity owing to the uniform dispersion and inclusion therein of colloidal silica. Nevertheless, it has been found that even by this method, the following problem remains yet to be solved.
The coating which is obtained by the method of Japanese Patent Application Laid-open No. SHO 60(1985)-39,169 is of the so-called non-rinse type which is obtained by applying a film-forming treating liquid on the surface of the metallic material and, without rinsing the coated surface, subjecting the coated metallic material to drying and baking. In the acrylic resin coating to which hydrophilicity is imparted by the addition of colloidal silica in accordance with this method, therefore, the amount of the residue of water-soluble hexavalent chromium compound from the treating liquid is so large as to be harmful to humans. When the metallic material treated by this method is used for the cooling fins of an air conditioner, for example, the possibility exists that the hexavalent chromium salt from the coating may dissolve into the water condensed from the ambient air and creates a pollution problem. Moreover, while the coating obtained by this method endures much longer in use than the hydrophilic coating having colloidal silica simply deposited on a chemical undercoat, because the colloidal silica is dispersed and retained fast within the layer of acrylic acid resin, this coating nevertheless is slightly inferior in the degree of initial hydrophilicity.